The invention relates to the application of heat to a moving web of material and, in particular, to an apparatus and method for preheating a paper web to precondition prior to combination in a corrugated paperboard web.
In the manufacture of corrugated paperboard, the various component paper webs are usually preheated prior to the application of adhesive and the joining of one web to another. A common preheating device in a corrugator comprises a rotating steam heated roll around which the moving paper web is partially wrapped and heated before it is transferred to downstream processing equipment, such as a single facer or a double backer.
More recently, it has been found that web preheating devices may be substantially simplified and the heat transfer to the web provided more efficiently by utilizing a vacuum-assisted stationary hot plate over which the web travels. U.S. Pat. No. 5,600,900 entitled "Vacuum-Assisted Web Drying System" by Carl R. Marschke, issuing on Feb. 11, 1997 and assigned to the assignee of the present application as well as copending patent application entitled "Edged Seal for Vacuum Preheater" by Carl R. Marschke, filed on Jun. 1, 1998, Ser. No. 09/088,214 are examples of vacuum-assisted stationary hot plates for web drying. In practice, such hot plates have comprised a plurality of rectangular steam tubes that are arranged to form a generally planar web supporting surface with a plurality of narrow vacuum channels between the vacuum tubes. Although such vacuum hot plates are much simpler in construction than rotary preheating rolls, certain processing problems have arisen. For example, paper dust can clog the channels which reduces the effectiveness of the vacuum, thereby reducing heat transfer. In addition, it has been found that baggy lateral edges of the paper web often do not maintain sufficient surface contact with the heated surface to uniformly precondition the paper web.